1. Field of the Invention
The present invention relates to a laser apparatus, and more particularly it relates to an ultraviolet laser apparatus for generating ultraviolet light capable of suppressing generation of speckle with low coherence, such as an exposure light source used in a photo-lithography process for manufacturing micro devices such as semi-conductor elements, liquid crystal display elements, CCD and thin film magnetic heads, as well as relates to an exposure apparatus using such an ultraviolet laser apparatus.
2. Description of the Related Art
As information technology equipment has progressed, regarding integrated circuits, improvement in function, memory capacity and compactness have been requested, and, to achieve this, it is required that the degree of the integration of the integrated circuit be increased. In order to increase the degree of the integration, individual circuit patterns should be made smaller. However, a minimum pattern dimension of the circuit is generally determined by performance of an exposure apparatus used in a circuit manufacturing process.
In an exposure apparatus utilizing photo-lithography, a circuit pattern exactly described on a photo-mask is optically projected and transferred, with reduced scale, onto a semiconductor wafer on which photoresist is coated. A minimum pattern size (resolving power) R on the wafer in the exposure is represented by the following equation (1) and the depth of focus DF is represented by the following equation (2) when it is assumed that a wavelength of a light source used for projection in the exposure apparatus is xcex and a numerical aperture of a projection optical system is NA:
R=Kxc2x7xcex/NAxe2x80x83xe2x80x83(1)
DF=xcex/{2xc2x7(NA)2}xe2x80x83xe2x80x83(2)
where, K is a constant.
As apparent from the above equation (1), in order to decrease the minimum pattern size R, the constant K may be decreased or the numerical aperture NA may be increased or the wavelength xcex may be decreased.
Here, the constant K is a constant determined by the projection optical system or process and normally has a value of about 0.5 to 0.8. A method for decreasing the constant K is referred to as a super-resolution technique in a broader sense.
Regarding such a technique, an improvement in the projection optical system, modified illumination and a phase shift mask method have been proposed and investigated. However, they had disadvantage that applicable patterns were limited. On the other hand, from the above equation (1), the greater the numerical aperture NA the smaller the minimum pattern size R. However, this also means that the depth of focus is decreased, as apparent from the above equation (2). Thus, there is a limit to increase the numerical aperture NA, and, in consideration of the balance between NA and DF, the value of the numerical aperture NA is normally selected to about 0.5 to 0.6.
Accordingly, a most simple and effective method for decreasing the minimum pattern size is a method for decreasing the wavelength xcex used in the exposure. There are several conditions in achieving reduction of the wavelength and in manufacturing the light source of the exposure apparatus. Now, these conditions will be described.
In a first condition, light output of several watts is required for shortening a time period for exposing and transferring the integrated circuit pattern.
In a second condition, in case of ultraviolet light having a wavelength smaller than 300 nm, material used for forming a lens of the exposure apparatus is limited, and it is difficult to correct chromatic aberration. Thus, monochromaticity of the light source is required and spectral width of must be smaller than 1 pm.
In a third condition, as spectral width is made narrower, temporal coherence is increased. Therefore, if light having a narrow line width is emitted as it is, an undesired interference pattern called as speckle will be generated. Accordingly, in order to suppress occurrence of the speckle, the spatial coherence in the light source must be reduced.
In order to satisfy these conditions and to realize high resolving power, many attempts for decreasing the wavelength of the exposure light source have been made. Heretofore, reduction of the wave length has been investigated mainly in the following two ways. One way is a development to apply an excimer laser having a short oscillation wavelength to the exposure apparatus, and the other way is a development of a short wavelength exposure light source utilizing harmonic wave generation from an infrared or visual laser.
Among them, as the short wavelength light source realized by using the former way, a KrF excimer laser (wavelength of 248 nm) is known, and, nowadays, an exposure apparatus using an ArF excimer laser (wavelength of 193 nm) as a shorter wavelength light source is being developed. However, these excimer lasers have several disadvantages that they are bulky, that optical parts are apt to be damaged because of great energy per one pulse and that maintenance of the laser is troublesome and expensive because of usage of harmful fluorogas.
On the other hand, as the latter way, there is a method for converting long wavelength light (infrared light or visual light) into shorter wavelength ultraviolet light by utilizing secondary non-linear optical effect of non-linear optical crystal. For example, in the document xe2x80x9cLongitudinally diode pumped continuous wave 3.5 W green laserxe2x80x9d (L. Y. Liu, M. Oka, W. Wiechmann and S. Kubota, Optic Letters, vol. 19 (1994), p 189), a laser light source for wavelength-converting light from a solid-state laser of semiconductor excitation type is disclosed. In this conventional example, a laser beam having a wavelength of 1064 nm and emitted from an Nd:YAG laser is wavelength-converted by using the non-linear optical crystal to thereby generate 4th harmonic light having a wavelength of 266 nm. Further, the xe2x80x9csolid-state laserxe2x80x9d is a general term of lasers in which a laser medium is solid. Accordingly, although a semiconductor laser is included in the solid-state laser in a broad sense, normally, the solid-state laser means lasers excited by light such as a Nd:YAG laser and a ruby laser, and, thus, in this specification, such a definition is used.
Further, as an example that the solid-state laser is used as the light source of the exposure apparatus, an array laser in which a plurality of laser elements each comprising a laser generating portion for generating a laser beam and a wavelength converting portion for wavelength-converting the light from the laser generating portion into ultraviolet light are bundled in a matrix patterns has been proposed. For example, Japanese Patent Laid-open No. 8-334803 (1996) discloses an example of an array laser in which a plurality of laser elements for wavelength-converting light from a laser generating portion having a semiconductor laser into ultraviolet light by using non-linear optical crystal provided in a wavelength converting portion are bundled in a matrix pattern (for example, 10xc3x9710) to thereby form a single ultraviolet light source.
According to the array laser having the above-mentioned arrangement, by bundling the plurality of independent laser elements together, light output of the entire apparatus can be increased while keeping light output of the individual laser element at a lower level. Thus, the load to the non-linear optical element can be reduced. However, since the laser elements are independent, when they are applied to the exposure apparatus, as a whole, oscillation spectra of the laser elements must be coincided. For example, even when the line width of the oscillation spectrum of each laser elements is smaller than 1 pm, the difference in relative wavelength in the entire assembly including the plural laser elements must not be 3 pm, and the entire width must be smaller than 1 pm.
To achieve this, for example, lengths of resonators of the laser elements must be adjusted or wavelength selecting elements must be inserted into the resonators so that the laser elements each can independently perform single longitudinal mode oscillation having the same wavelength. However, these methods have disadvantages that the adjustment is delicate and that, as the number of laser elements is increased, the arrangement for causing all of the laser elements to perform oscillation having the same wavelength becomes more complicated.
On the other hand, as a method for actively equalizing the wavelengths from the plurality of laser elements, an injection seed method is well known (for example, refer to a document xe2x80x9cSolid-state Laser Engineeringxe2x80x9d, 3rd Edition, Springer Series in Optical Science, Vol. 1, Springer-Verlag, ISBN 0-387-53756-2, p 246-249 presented by Walter Koechner). This method is a technique in which light from a single laser light source having narrow oscillation spectrum line width is branched to a plurality of laser elements and oscillation wavelengths of the laser elements coincide or are tuned by using the laser beams as seed light, thereby making the line widths of the spectra narrower. However, this method has a disadvantage that the arrangement becomes complicated, since an optical path for branching the seed light into the laser elements and a tuning and controlling portion for the oscillation wavelengths are required.
Further, although such an array laser can make the entire apparatus smaller considerably in comparison with the conventional excimer lasers, it is still difficult to obtain a packaging capable of suppressing output beam diameter of the entire array to less than several centimeters. Further, in the array laser having such an arrangement, there arise problems that the laser is expensive because the wavelength converting portions are required for the respective arrays and that, if mis-alignment occurs between the laser elements constituting the array or if the optical element(s) are damaged, in order to adjust the laser elements, the entire array must once be disassembled to remove the laser elements and the removed array must be assembled again after adjustment thereof.
The present invention aims to eliminate the above-mentioned disadvantages and problems, for example, problems regarding bulkiness of the apparatus, usage of harmful fluorogas and troublesome and expensive maintenance which are caused when the excimer laser is used as the ultraviolet light source of the exposure apparatus, problems regarding the damage of the non-linear optical crystal and the occurrence of the speckle due to increase in the spacial coherence which are caused when the harmonic wave of the solid-state laser such as the Nd:YAG laser is used as the ultraviolet light source of the exposure apparatus, and problems regarding the complexity of the construction including the wavelength-coincidence mechanism, the difficulty in reduction of diameter of the output beam and the troublesome maintenance which are caused when the array laser in which the plurality of laser elements for generating the ultraviolet light are bundled in the matrix pattern is used as the ultraviolet light source of the exposure apparatus.
Accordingly, an object of the present invention is to provide an ultraviolet laser apparatus in which ultraviolet light having a single wavelength and sufficiently narrower bandwidth for a light source of an exposure apparatus can stably be obtained as ultraviolet output light having low spatial coherence and which is compact and easily handlable.
Another object of the present invention is to provide an exposure apparatus which is compact and has high degree of freedom and in which such an ultraviolet laser apparatus which is compact and easily handlable is used as a light source.
The above object is achieved by an ultraviolet laser apparatus comprising a laser generating portion for generating light having a single wavelength, at least one stage optical amplifier having a fiber optical amplifier for amplifying the generated laser light, and a wavelength converting portion for wavelength-converting the light amplified by the optical amplifier into ultraviolet light by using a non-linear optical crystal.
More specifically, the laser generating portion includes a single-wavelength oscillating laser (for example, a DFB semiconductor laser 31 in an embodiment and the like) having a narrow band, and the laser light having the single wavelength is amplified by the fiber optical amplifier (for example, an erbium doped fiber optical amplifiers 33, 34 in an embodiment and the like), and the output light from the fiber amplifier is converted into an ultraviolet light (for example, ultraviolet light having a wavelength of 193 nm or 157 nm) by the wavelength converting portion using the non-linear optical crystal (for example, crystals 502 to 504 in an embodiment and the like). In this way, an ultraviolet laser apparatus which is compact and easily handlable and which is adapted to generate ultraviolet light having a single wavelength and which constitutes the object of the invention is provided.
Further, in the present invention, the output from the single-wavelength oscillating laser (for example, DFB semiconductor lasers 11,21 or a fiber laser in an embodiment and the like) is divided or branched by a light dividing or branching device. The output is divided into plural outputs by the light dividing or branching device (for example, splitters 14, 16 in an embodiment and the like), and the fibers are arranged behind the device, and, by bundling the plurality of fibers, the ultraviolet laser apparatus is formed. Further, the light dividing or branching device may have any design so long as the laser light generated by the single-wavelength laser can be divided or branched in parallel.
By providing a device for preventing the branched light beams to be overlapped in view of time, independent light beams can be obtained. A preferred device for achieving this may comprise beam splitters for branching the laser light generated by the single-wavelength laser into plural light beams in parallel, and fibers having different lengths and disposed at output sides of the beam splitters. In a preferred form of the fibers having different lengths, the lengths of the fibers are selected so that, after the laser beams branched in parallel pass through the respective fibers, delay distances or intervals between the successive laser beams becomes substantially the same at the output ends of the fibers.
Further, according to one aspect of the present invention, as the light dividing or branching device, a time division multiplexer (TDM) (for example, a TDM 23 in an embodiment) for distributing the light into respective light paths every predetermined time is used.
Next, as the plurality of fibers disposed on the output sides of the light dividing or branching device, a plurality of fiber optical amplifiers are preferable. Further, it is preferable that the light beams are amplified by the fiber optical amplifiers (for example, erbium doped fiber optical amplifiers or ytterbium doped fiber optical amplifiers 18, 19 in an embodiment and the like) and the plurality of fiber optical amplifiers are bundled. With this arrangement, laser light having higher intensity can be obtained. Further, if necessary, non-doped fibers may be coupled to the output ends (for example, fiber output ends 114, 29 in an embodiment and the like) of the plurality of fiber optical amplifiers.
In the fiber output ends, it is preferable that diameters of cores (for example, a core 421 in the drawing of an embodiment) of the fibers are gently diverged in a tapered fashion toward the output end faces. Further, it is preferable that a window member (for example, window members 433, 443 in an embodiment and the like) which are transparent to the laser light is provided on the fiber output ends. With this arrangement, power density (light intensity per unit area) of the laser light can be reduced at the fiber output ends, and, accordingly, the fiber output ends can be prevented from being damaged.
Further, according to the present invention, in a plurality of fibers provided on an incident side of the wavelength converting portion, it is preferable that output ends of the fibers are bundled as a single bundle or plural bundles and are formed as a single output group or plural output groups (for example, bundle outputs 114, 29, 501, 601, 701 in an embodiment and the like) to match with the construction of the wavelength converting portion. And, in the wavelength converting portion, harmonic wave generation for fundamental wave is effected by one set or plural sets of non-linear optical crystals (for example, crystals 502 to 504 in a fourth embodiment or crystals 842 to 844 in an eighth embodiment) to output ultraviolet light (for example, ultraviolet light having a wavelength of 193 nm or 157 nm). By providing one set of wavelength converting portions, the apparatus can be made more compact and cheaper, and, by providing the plural sets of wavelength converting portions, since the load acting on each set can be reduced, high output can be achieved as a whole.
Further, when the optical amplifier is constituted by the plurality of fiber optical amplifiers, in order to suppress fluctuation in ultraviolet light outputs due to fluctuation in amplification gain of the fiber optical amplifiers, it is desirable to provide a fiber output control device for monitoring the output lights from the fibers to control pumping power of the fiber optical amplifiers. Further, in order to equalize light wavelengths of the ultraviolet light outputs to a specific wavelength, it is desirable to provide a control device for controlling the oscillating wavelength of the single wavelength oscillating laser by using the frequency of fundamental wave or the harmonic wave in the wavelength converting portion.
Light collecting optical elements are provided at the incident sides of the wavelength converting portions. Application of the light collecting optical elements can be appropriately selected in accordance with the output conditions of the optical amplifiers. For example, according to one aspect, the light collecting optical elements (for example, lenses 902, 453 in an embodiment and the like) may be provided for each fiber output, or, according to another aspect, the light collecting optical elements (for example, lenses 845,855,463 in an embodiment and the like) may be provided for each of the bundled output groups.
By the way, as an arrangement for outputting the ultraviolet light, for example, there is a technique in which a laser beam having a wavelength of about 1.5 xcexcm is emitted from the laser light generating portion, and, regarding the optical amplifier, at least one stage optical amplifier having the fiber optical amplifiers for amplifying the fundamental wave having a wavelength of about 1.5 xcexcm is provided, and, a wavelength converting portion for effecting 8th harmonic wave generation for the amplified fundamental wave is also provided. With this arrangement, the ultraviolet light having output wavelength of about 190 nm can be generated. The output light can be made to have the same wavelength as the wavelength (of 193 nm) of the ArF excimer laser by tuning the oscillating wavelength of the laser light generating portion more finely (for example, 1.544 to 1.522 xcexcm).
Further, as another arrangement for outputting the ultraviolet light, for example, there is a technique in which, similar to the above-mentioned arrangement, a laser beam having a wavelength of about 1.5 xcexcm is emitted from the laser light generating portion, and, regarding the optical amplifier, at least one stage optical amplifier having the fiber optical amplifiers for amplifying the fundamental wave having a wavelength of about 1.5 xcexcm is provided, and, a wavelength converting portion for effecting 10th harmonic wave generation for the amplified fundamental wave is also provided. With this arrangement, the ultraviolet light having output wavelength of about 150 nm can be generated. This output light can be made to have the same wavelength as the wavelength (of 157 nm) of the F2 laser by tuning the oscillating wavelength of the laser light generating portion more finely (for example, 1.57 to 1.58 xcexcm).
Further, as a further arrangement for outputting the ultraviolet light, for example, there is a technique in which a laser beam having a wavelength of about 1.1 xcexcm is emitted from the laser light generating portion, and, regarding the optical amplifier, at least one stage optical amplifier having the fiber optical amplifiers for amplifying the fundamental wave having a wavelength of about 1.1 xcexcm is provided, and, a wavelength converting portion for effecting 7th harmonic wave generation for the amplified fundamental wave is also provided. With this arrangement, the ultraviolet light having output wavelength of about 150 nm can be generated. The output light can be made to have the same wavelength as the wavelength (of 157 nm) of the F2 laser by tuning the oscillating wavelength of the laser light generating portion more finely (for example, 1.099 to 1.106 xcexcm).
Further, as the other arrangement for outputting the ultraviolet light, for example, by providing a laser light generating portion including a semiconductor laser or a fiber laser having oscillating wavelength of about 990 nm, at least one stage optical amplifier having the fiber optical amplifiers for amplifying the fundamental wave having a wavelength of about 990 nm, and a wavelength converting portion for effecting 4th harmonic wave generation for the amplified fundamental wave, the ultraviolet light having the same wavelength as the wavelength (of 248 nm) of the KrF excimer laser can be obtained.
The wavelength converting portion for effecting such a harmonic wave generation can be designed as various arrangements as will be described in embodiments of the present invention which will be described later. For example, briefly explaining an example of an arrangement of the wavelength converting portion for effecting 8th harmonic wave generation for the fundamental wave, such an arrangement can be constituted by a three-stage harmonic wave generation light path system (for example, FIG. 11(a) in a fourth embodiment) for converting the fundamental wave into 2nd harmonic wavexe2x86x924th harmonic wavexe2x86x928th harmonic wave by utilizing second harmonic wave generation (SHG) of the non-linear optical crystal in all of the wavelength converting stages. With this arrangement, a desired 8th harmonic wave can be obtained with least number of stages.
Further, as another preferred arrangement for obtaining the 8th harmonic wave, there is an arrangement (for example, FIG. 11(d) in a fourth embodiment and the like) in which 3rd harmonic wave and 4th harmonic wave of the fundamental wave are generated by also utilizing sum frequency generation (SFG) of the non-linear optical crystal in the wavelength converting stages, and these waves are subjected to sum frequency generation to generate 7th harmonic wave of the fundamental wave, and further, the 7th harmonic wave and the fundamental wave are subjected to sum frequency generation to generate 8th harmonic wave of the fundamental wave. In this arrangement, a LBO crystal having low absorption coefficient for ultraviolet light having the wavelength of 193 nm can be used for 8th harmonic wave generation in the last stage. Further, regarding 7th harmonic wave generation and 10th harmonic wave generation from the fundamental wave, similar to 8th harmonic wave generation from the fundamental wave, the second harmonic wave generation and the sum frequency generation of the non-linear optical crystal can be used.
Further, according to an aspect of the present invention, ultraviolet pulse laser light can be obtained by providing a pulsing device (for example, light modulating elements 12, 22 in an embodiment and the like) for pulsing CW laser light of the single wavelength oscillating laser in the laser generating portion or by pulse-oscillating the single wavelength oscillating laser or by doing both. Further, by using the ultraviolet laser apparatus having the above-arrangement as a light source of a projection exposure apparatus, and by providing an illumination optical system for illuminating light from the light source onto a mask on which a projection pattern is printed with substantially uniform intensity and a projection objective optical system for projecting the pattern printed on the mask onto a wafer, a projection exposure apparatus having easy maintenance can be obtained.
As mentioned above, according to the present invention, since the light having single wavelength from the laser generating portion is amplified by the optical amplifier in the light source and the amplified light is converted into the ultraviolet light by the non-linear optical crystal of the wavelength converting portion, the ultraviolet light having the desired spectrum line width (for example, 1 pm or less) can easily be obtained without complicated arrangement.
Further, since the laser light having single wavelength is divided (or multiplexed) into the plurality of output lights and the plural output lights are amplified by the plurality of fiber optical amplifiers and the amplified lights are converted into the ultraviolet lights by the non-linear optical crystals, the entire laser light output can be increased while suppressing the peak power of the pulse light per one pulse and the ultraviolet light having low spatial coherence can be obtained.
That is to say, according to the present invention, there can be provided an ultraviolet laser apparatus which is compact and has high degree of freedom regarding arrangement of parts and in which the maintenance is easy and the non-linear optical crystal is hard to be damaged and the spatial coherence is low.